Many energy-using devices heretofore operated from conventional electrical wall sockets are being adapted to operate from energy supplied by batteries comprised of one or more electrochemical cells. Such adaptation provides such energy-using devices with the feature of portability; that is to say, the device has a self-contained energy source and may be operated at locations remote from electrical wall sockets. Further adaptation of energy-using devices for compatability with rechargeable type batteries provides additional convenience for the user especially if the device includes means for recharging of the batteries.
In some energy-using devices powered by a battery, it is critically important to notify the user of the device that the battery is nearly depleted of energy. If so notified, the user may use the remaining energy to operate the device to complete various tasks before the device becomes inoperable for lack of sufficient power. For example, a portable computer is one device where such notification is essential. A user of a portable computer needs to be forewarned of imminent exhaustion of battery energy at a point in time when a specific amount of energy is still available from the battery. Such a warning may be necessary, for example, to enable the user to the portable computer to transfer files and data, stored in the volatile working memory of the portable computer, to a permanent memory such as a tape or a disk before the deliverable battery energy is depleted. If the user is not provided with sufficient energy to accomplish this task, the files and data stored in the volatile memory of the personal computer will be lost.
One approach that has been used to ensure that the user of a portable computer has sufficient energy remaining to accomplish a specific task is to simply supply a second back-up battery to provide powering of the device once the first battery is depleted. In the past, this approach has not proved to be entirely commerically satisfactory since the addition of a back-up battery to the system reduces the system's reliability, requires additional charging means, adds additional cost to the charging system and requires that additional space in the portable computer be made available to house the battery. For example, a portable computer using a rechargeable 12 volt battery would require ten rechargeable nickel-cadmium cells each providing 1.2 volts. A back-up battery providing the same 12 volt energy supply would also require ten individual cells. The increased cost, added space requirement and decrease in reliability introduced into the battery supply system by the addition of a ten cell back-up battery makes this solution to the problem commercially unacceptable in many applications.
Another approach that has been used to ensure that the user of a portable computer has sufficient energy available to accomplish a specific task involves measuring the amount of energy delivered by the battery and warning the user when the measured delivered energy reaches a predetermined value. This approach involves integration of the charge added and charge removed from the battery and, accordingly, is commercially unattractive due to system complexity and cost.
Another approach used in the prior art with batteries having appropriate discharge characteristics, involves measuring a discharge parameter of the battery to obtain an indication of the amount of energy remaining in the battery and then warning the operator of an energy using device when the energy has been depleted to a low level. This approach is unsuited for some batteries, for example, nickel-cadmium batteries. While nickel-cadium rechargeable batteries are well adapted to provide energy to many portable energy-using devices, a nickel-cadmium battery does not have a discharge parameter the measurement of which would provide a reliable indication of the level of remaining energy in the battery. Typically, a nickel-cadmium battery is usually comprised of a plurality of rechargeable nickel-cadmium cells of the same deliverable energy capacity electrically connected in series. A battery so comprised provides a substantially constant voltage during battery discharged. As the battery is discharged, the same amount of energy is removed from each of the cells. Thus, near the end of total discharge of the battery all of the energy from each cell has been depleted and the previously constant voltage falls at an extremely rapid rate below a voltage which may not properly operate the energy-using device. Because the voltage of a nickel-cadmium battery remains constant until such time as the energy has been depleted from each of its cells, it is very difficult, if not practically impossible to use the voltage of the battery to detect the imminent depletion of energy of the cell and yet ensure that sufficient energy remains to perform a specific task.